The way Donal Doherty tells it, the most remarkable thing about his restaurant is not that it has won the OFM award for Best Newcomer. It’s that it exists at all. “It was a terrible December day, when I first saw the building, with the rain coming down and the sea that stormy,” Doherty says now of Harry’s Shack, which overlooks the tumble and boil of the Atlantic at Portstewart, an hour’s drive north of Belfast. “There was no glass in the windows, and sand piled up inside everywhere.” He concluded that to take it on would be either “mad” or “genius”.

Almost two years on and it turns out it was genius. After a six-figure refit, and a menu devised by chef Derek Creagh, Harry’s Shack has established itself as a shining star of Northern Ireland’s fast-emerging food scene. As the name suggests, it is a long low-slung wooden shed, owned by the National Trust and once used as the beach office. Now there are picture windows framing the glorious views, a wood burning stove and a fish-heavy menu, boasting the best the day boats can deliver.

It’s the place for perfect whitebait with their own Marie Rose sauce, or sweet caramelised onion tart with curls of salty local cheese. It’s the place for fillets of gurnard with ratatouille and couscous, classic fish and chips, bronzed roast chicken legs with macaroni in a truffle cream sauce, or a huge hamburger kept from toppling over by a steak knife straight down the middle. “It’s just solid non-fussy food built on great ingredients,” says Creagh, who worked with Heston Blumenthal at the Fat Duck, and at Chez Bruce in London, before returning to Northern Ireland and Deanes in Belfast. Right now it’s bring-your-own, but there’s a good wine shop nearby. They are in the process of applying for a licence.

The Harry in the shack’s name is Donal’s father, who opened a restaurant in Donegal 25 years ago. Thirteen years ago Donal gave up his career as an accountant in London and returned to take it over. “It had been hugely successful for a decade but by that point it wasn’t in a good way,” he says now. “Too much of what we were using was frozen.” To turn things around he planted a two-acre plot with 140 varieties of fruit and veg to service the restaurant, began buying fish off the day boats, and brought a beef-hanging room back into use.

“But the volume of trade wasn’t right,” Donal says. And so began the search for a second business, answered by the National Trust, when they showed him the disused shed. “I have to say the National Trust have been brilliant all down the line. Originally they suggested a tea room but I knew I wanted to do more and they went with it.”

The idea of a restaurant on a beach isn’t entirely original. But they tend to be seasonal. Harry’s Shack is open year round, and it uses Facebook to warn people when the storms have got the better of them. It doesn’t happen often.

So what does Donal think is the appeal? “When people come here they’ve usually parked on the beach and already walked on the sand. By the time they come through the door they’ve already had a good time.” And boy, have they come. The business plan was based on 50 people for lunch and 40 for dinner. In the summer they’re getting more than three times that. Even in winter they’re seeing 150 people a day. Did he think it would be so successful? “Of course not. We were so worried about breaking even, we didn’t think about anything else.”

Donal acknowledges that social media has served them well. “We post pictures of sunsets and people click ‘like’ in their thousands.” And then they start phoning to book. And then they get in their cars and drive up. After all, who doesn’t want to eat cracking food while watching the sun go down over a stormy, gunmetal sea?

118 Strand Road, Portstewart; 028 7083 1783; facebook.com/HarrysShack